Sitting down to do hours of work is hard. But there is an alternative. Try sitting down for a few minutes of intense focus on a concept or topic you need to revise. You will find that this is less daunting and that it actually gives you the momentum to keep going for hours! Try it. Give yourself 3 minutes (or the length of a song) of intense focus on a subject, and see how much easier it is to revise for hours on end.